configure: fix check for valid compiler flags with Clang

By default Clang reports unknown compiler flags as warnings
thus the valid compiler flags check fails to work properly.
Clang needs to have -Werror passed.

configure.ac

@@ -156,12 +156,12 @@ if test -n "${MAINTAINER_MODE_FALSE}"; then
 fi
 
 all_desired_work=false
-AS_COMPILER_FLAG([$DESIRED_FLAGS], [all_desired_work=true])
+AS_COMPILER_FLAG([-Werror $DESIRED_FLAGS], [all_desired_work=true])
 if $all_desired_work ; then
     CFLAGS="$CFLAGS $DESIRED_FLAGS"
 else
 for flag in $DESIRED_FLAGS; do
-  AS_COMPILER_FLAG([$flag], [CFLAGS="$CFLAGS $flag"])
+  AS_COMPILER_FLAG([-Werror $flag], [CFLAGS="$CFLAGS $flag"])
 done
 fi